Mercury occurs in the subsurface at a Superfund site in New York. The United States Environmental Protection Agency (USEPA) approved a remedy which includes in-situ stabilization/solidification (ISS). A Treatability Study (TS) was conducted to support the full-scale design, with the objective of identifying stabilizing and solidifying agents to achieve the groundwater criteria and soil strength requirements. The leachability was assessed using the Synthetic Precipitation Leaching Procedure (SPLP). The soil strength was assessed by Unconfined Compressive Strength (UCS) tests on treated samples. Aliquots were tested for hydraulic conductivity to evaluate the post-ISS reduction in groundwater flow rate. Volumetric expansion tests and Toxicity Characteristic Leaching Procedure (TCLP) tests were conducted to assess disposal options for excess (swell) soils. The TS concluded that a mixture of Portland cement/calcium polysulfide (CP) was effective in achieving the leachability and soil strength objectives. SPLP concentrations were on the order of 0.001-0.010 milligrams per liter (mg/L) for the entire range of the CP addition rates of 1.5-5% of the wet soil weight (w.w.), indicating that there is no advantage in increasing the dosage. These values were one to two orders of magnitude lower than those obtained for the untreated soils. The UCS increased with the cement addition. The hydraulic conductivity was a function of cement content; a 10-100 fold decrease versus the in-situ values was achieved with the addition rates of 3-4% w.w. Because of the high mercury content and high TCLP, the anticipated installation scheme includes applying the ISS from the original grade and subsequent offsite disposal of the swell, as opposed to the typical excavation/disposal of the top layer prior to the ISS.